1. Field of the Invention
The present invention relates to a resonator of the string or beam type vibrating under flexion and comprising fixing means at at least one end and inertia mass vibratory decoupling means disposed between the string and the fixing means for preventing mechanical vibratory energy from being transferred between said string and its fixing means.
2. Description of the Prior Art
Resonators of this type are excited and kept oscillating by different means, for example electromagnetic, electrostatic, piezoelectric, on one of their natural frequencies.
Such resonators are used as sensitive elements for forming:
force, pressure, acceleration or temperature sensors when they are subjected to axial forces,
stable oscillators,
gyrometers or gyroscopes.
The final qualities of such devices are all the greater the better the confinement of the mechanical vibratory energy in the string , i.e. the more the dissipation of energy to the mounts is reduced. This may be obtained by defining efficient decoupling structures avoiding draining off by the supports of a part of the mechanical energy.
For that numerous resonators have already been proposed.
One of the first mountings proposed, such as described in the U.S. Pat. No. 3 479 536 (NORRIS), comprises no vibratory decoupling element but, on the other hand, provides for thinned down portions of the fixing elements allowing rotation about axes parallel to the vibration plane for reducing the non axial stresses which might be created because of a poor alignment of the supports.
The same patent also teaches vibratory decoupling using a method of fixing the vibrating string by its nodal lines. This reduces energy losses. The connecting points are reduced to a minimum between the string and the supports at the level of the nodal lines, by a special cutting of the resonator. This material connection nevertheless introduces an inevitable angular stiffness and thus causes and transmits torques at the level of the nodal lines, which causes an exchange of energy with the outside resulting in a certain vibratory energy loss. Moreover, this type of resonator has the disadvantage, when it is used for force sensors, that the forces transmitted to the string from fixing zones provided at each end are not distributed homogeneously over the whole of the string, which causes appreciable movements of the nodal vibration points, depending on the forces exerted and so deteriorates the vibratory decoupling.
It is also known to form symmetric resonators, in the form of a tuning fork, constituted by two identical strings. In theory this configuration is perfect but in practice it requires adjustments of form and mass which it is only possible to provide for a fixed frequency resonator and which cannot be envisaged for resonators having a large range of usable frequencies. Such a construction is proposed in the U.S. Pat. No. 4 215 570 (NISSE).
Finally, another decoupling means may be obtained by inertia effect. It consists in inserting inertia masses between the string and its fixing ends. Decoupling of this type is described in the U.S. Pat. No. 3 470 400 (WEISBORD), which recommends using two U shaped inertia masses at each end of the string. These masses are further each connected to fixing means by means of two flexible blades, functioning in a deformable parallelogram. Thus the decoupling is obtained by the translational movement of the masses following a reciprocal movement in the vibration plane of the string and in phase opposition with respect thereto.
Such decoupling, with complex geometry, involves very reduced dimensional tolerances for eliminating the complex parasite movements of the masses. Now these movements result in compression and extension forces on the blades of the deformable parallelogram and so in appreciable energy losses, transmitted to the fixing points. To attenuate the forces applied to the connecting blades it has been proposed to artificially reduce the stiffness thereof under traction-compression, by modifying the structure, for example, by inserting between the blades and the masses other blades perpendicular to the first ones.
The construction of such transducers involves machining, carried out more particularly by ultrasonic techniques, which requires several successive operations in which different tools are used for rough machining the blanks, machining the shapes and adjusting the sizes. These operations, which require previous adjustments for each of the tools, leads to a very high manufacturing cost.